The requirements to be met by the dimensional and configurational accuracy of cold-rolled thin-gage strip have considerably increased during the course of the development of rolling technique and further processing. An ideal cold-rolled strip not only is to exhibit the same thickness over length and width, but also is to lie completely planar. In this connection, planeness is to be preserved even if the strip is cut into sections during further processing.
These requirements with respect to dimensional accuracy and planeness of a thin-gage strip cannot be met, however. For example, if an attempt is made to cold-roll a hot-rolled strip, somewhat thinner along the edges than in the center, so that its thickness is completely the same over its width, this requires a larger reduction in thickness and thus greater stretching in the center of the strip, leading to the formation of central waviness. In contrast, if the best possible planeness is desired, this is achieved at the cost of transferring the profile shape of the hot-rolled strip to the cold-rolled strip.
Flaws in planeness can appear after rolling or as late as during the subsequent further processing. During rolling, flaws in planeness are essentially a consequence of differing stretching over the strip width on account of non-uniform shaping of the strip in the roll nip over the strip width. During further processing, for example during slitting, flaws in planeness frequently occur by the triggering of natural stresses produced during rolling.
One differentiates between flaws in planeness that can be levelled by stretching and those which cannot be levelled by stretching. Capable of being levelled by stretching applies to those flaws where the strip deviates uniformly from planeness in the width direction. In this case, mutually opposed natural stresses occur on the topside and on the underside of the strip. These stresses are constant over the entire width. Unevennesses that can be levelled by stretching are characterized in that they are linearly delimited in one direction, i.e. in the longitudinal direction or in the transverse direction.
Deviations in planeness variable over the strip width and length are characterized by curved boundaries and cannot be stretched level by means of a simple bending process. In this case, non-uniform natural stress distributions are present in the longitudinal and transverse directions. Such planeness flaws appear as central and marginal waviness in the cold-rolled strip.
During the cold rolling of strips of steel or aluminum, differences in length and/or the differing stretching over the strip width are at least partially compensated by the elastic elongation on account of strip tension so that there is no unequivocal criterion, in the rolling procedure, for unduly high strip tensions, especially in the marginal zones of the strip, which lead to strip fissures. This lack of information can have the result that the rolling efficiency existing in many rolling mills is not exploited and thus economical operation is not ensured.
The high requirements regarding the quality of rolled thin-gage strip have resulted in the development of measuring units to detect the planeness during cold rolling, permitting, together with the adjusting procedures for the roll nip, the building up of a closed control circuit for planeness, by means of which the tensile force exerted on the strip over the strip width in the outlet of a set of rolls can be regulated only with approximate constancy.
The planeness measuring roller utilized in the device for detecting and regulating the planeness of thin-gage strip exhibits the drawback that the tensile stress distribution in the strip, as a characteristic value for planeness, is measured in the individual strip fibers and/or strip zones in the edge regions of the strip only very inaccurately so that planeness control of this known device operates very imperfectly.